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R65 with r90 engine concerns

I'm looking seriously at an R65 which has a replacement engine from an R90. I've owned a bunch of BMWs but never one with a different model engine replacement and have a few questions. I think I know that the R65 has a lower final gear ratio but does this ratio difference happen in the rear end or in the transmission. Also, is the ratio difference significant enough to be a concern? Are there any other issues that I should be aware of? I'd appreciate any advice that you have before I make the purchase. Thanks. Jim

According to the specs, the transmission gear ratios are the same in the R65 and R90/6. An R90S had an optional sport transmission with different ratios. The rear end is what makes the difference. Note that the speedometer must be consistent with the rear end ratio to have a proper speed readout.

R65 with R90 engine

Thanks for the very quick responses. I've 3 days to make a decision and I haven't seen the bike in person yet so I'm gathering as much info as I can before the drive to see and ride it. I'm 70 yrs old and have owned 10 BMWs over the past 40 years and aggressive riding is now a thing of the past. However, I assume the 90 engine is heavier and with more torque so question whether the frame is up to this extra stress. I assume this to be no problem but it has been used as a highway commuter for several years and I would be using it for a little more twisty riding. I realize that, at any given speed, either engine would be spinning at the same RPM which only gives rise to questions regarding "buzz" points but adjusting travel speeds to accommodate these is an easy fix. I appreciate the reminder to look into speedometer issues, and I will, but my experience with BMWs is that speedometer readings aren't what one would consider to be extremely accurate anyway. On the other hand, if the speedometer sender is at the rear end, it should be ok because I'm assuming that the rear end is original R65 factory specs.

We have a club member who actually has an R60 with an R90 motor. This bike was owned by another member for years. The bike runs perfectly fine. The only thing that would concern me ever so slightly is the front drum brake. Not enough stopping power for a 900cc motor.

As Mike asks: what motor and frame/gearbox/rear end are we talking about?

Transplanting a post-1970 Airhead motor into a pre-1970 /2 frame is fairly common, but that recipient frame would be an R50 or R60, not an R65.

R65 machines were built 1979-1984 with frames a bit smaller than the then-current R75/R90/R100, and from 1985-1987 or so, with frames identically-sized as the larger-motor machines. The engine cases (i.e., bottom mount spacing, case width and height) on all the post-1970 Airheads are the same, so it'd be quite possible to drop an R90 motor into an R65 frame, although I've not heard of it being done.

R65

I've done this before with an R80 engine. It all fits together just fine if you're using an early R65 frame and the wiring is set up for a points ignition and/or you're using an aftermarket electronic ignition. the R90 engin uses 38mm exhaust, the R65 has 36mm. So if the bike is a running example, these things were taken into consideration.
If the R65 rear drive is used, then the speedo should match. If not, then you could send it to a speedo shop for re-calibration.
The R90 engine has the power to make it a very quick bike with the unfortunate loss of gas mileage. Add that to the smallish gas tank, and you're looking at more frequent stops for gas. When I built mine, I used the R80 rear drive (3.36 to 1) to improve mileage. Not a problem. It bolts right on. Then I put on a gas tank from an R100GS. If fits right on using the GS rubber pad on the bottom rear.

R65with R90 engine

Based on the comments here I feel comfortable buying this motorcycle and will unless, when I see it, it is a total rat cosmetically. In response to EMSimon's comments regarding the front brakes. The current owner said that it now has dual disks on the front wheel so it sounds like the person doing the mod did it right. I'll post results of what I find when I see it Friday. Thanks again.

The R65 actually has a beefier frame than the /6 or /7 bikes, so it is more than up to handling the bigger engine. Since the R90 engine is wider, you give up a tad of cornering clearance versus the original setup, which may not even matter depending how you ride.

No one has mentioned wheelbase ... the R65 is basically a short wheel base R75/5. Given the notorious handling characteristic of the SWB, does any of it translate to the R65. Don't know if the steering head geometry was changed so it may not be an issue.

edit - I found reference to the steering head angle being increased to 28?? on the 1971 /5 models (no ref. to what it was before the change). And reference to the R65 steering head angle being 28.85??.

Last edited by Lmo1131; 03-13-2013 at 08:59 PM.

"It is what you discover, after you know it all, that counts." _ John Wooden